MAN'S NEAR RELATIVES {MAMMALS). 231 
and a harder substance called enamel. In a human 
tooth the enamel covers entirely the outside of that 
part of the tooth known as the crown. The root, or 
fang, of the tooth is covered by a substance called 
cement. The greater part of the tooth is dentine. In 
the teeth of many animals which must grind their food 
for a long time the enamel and dentine are so folded 
together that hard sharp ridges of enamel are produced 
above valleys. When these valleys are deep they are 
filled with cement, as in the case of the molar teeth of 
Fig. 194.— Skull of Cow Showing the Bone of the Horn. 
the horse. As these teeth wear away the dentine and 
cement wear faster than the enamel and the ridges 
become more prominent. 
Gnawing animals, like the squirrel, have teeth fitted 
with enamel in front and dentine behind. As the teeth 
are used the dentine is worn away and a sharp chisel 
of enamel is produced. These teeth constantly increase 
in length from below, and, if they are not worn away 
fast enough, they sometimes grow so long that they 
cannot be used at all and the animal starves. In every 
